According to the Coriolis principle, whenever, in a system, a rotating mass movement and a straight line mass movement extending at least partially perpendicular to the rotational axis superimpose, an additional force always acts on the moved mass; this force is referred to as the Coriolis force. This effect is utilized in known manner in Coriolis flow measuring devices, for example, in order to determine mass flow of a medium flowing in a pipeline. During use, such Coriolis flow measuring devices are inserted, as so called inline-measuring devices, into a pipeline flowed through by the respective medium.
Frequently, measuring transducers are applied in Coriolis flow measuring devices which have two measuring tubes connected for parallel flow, between which a medium flowing in the pipeline is divided. As a rule, during use, the two measuring tubes are excited with opposite phase to one another. In this way, a decoupling of the oscillatory system, which has the two measuring tubes, from external vibration influences is achieved. Additionally, a measuring transducer of a Coriolis flow measuring device can also have more than two measuring tubes, such as, for example, four measuring tubes, which are connected for parallel flow relative to one another. In such case, in industrial applications, the situation can occur that in the case of such Coriolis flow measuring devices, (at least) one of the measuring tubes becomes completely or partially plugged. This case arises especially when a high-viscosity medium, an inhomogeneous medium and/or a medium tending to form accretions is flowing in the relevant pipeline.
Such a plugging of only a subset of the measuring tubes is difficult to detect, since a flow is still enabled through the at least one remaining free measuring tube. Even in the case of complete plugging of only one measuring tube, the performing of a Coriolis mass flow measuring with the Coriolis flow measuring device is possible. A plugging of a measuring tube is especially not directly recognizable based on the particular mass flow value. It is, however, desirable to detect a plugging of a measuring tube in Coriolis flow measuring devices as reliably and early as possible. This need exists especially in the case of hygienically critical applications and/or in the case of applications, in which the medium conveyed in the relevant pipeline changes, and a mutual contamination should be prevented.
In the publication WO 2009/134268 A1, a flow measuring device is described, by which a deviation in a flow measuring device parameter is detectable. Such a deviation of a flow measuring device parameter can, among other things, be triggered by a plugging of a measuring tube. In the case of a described method, the temperature is registered at a first as well as at a second measuring tube of the flow measuring device, and a temperature gradient is ascertained therefrom. A deviation in a flow measuring device parameter is detected when the ascertained temperature gradient exceeds a limit value. In the case of an additional described method, a pressure drop is measured via the flow measuring device, and based on this measuring, an expected flow rate is calculated. This expected flow rate is compared with a flow rate actually measured in the flow measuring device. A deviation in a flow measuring device parameter is then detected when the expected flow rate deviates from the measured flow rate by more than a limit value.